Carbon nanofibers are graphitic filament structures formed by their growth from the catalytic decomposition of a hydrocarbon in gaseous phase. This growth is determined by the kinetic and thermodynamic reaction conditions, the composition of the feedstock gas under high temperature conditions, and the nature of the metallic catalyst employed.
The various manufacturing methods for carbon nanofibers can be classified into two main groups, depending on whether the catalyst used is on a fixed substrate or whether it is a floating catalyst. They are also determined by the different reaction conditions and the composition of the working atmosphere, establishing different ranges in the parameters associated to each one.
The production process used to manufacture the carbon nanofibers is crucial from an industrial point of view, since the different conditions valid for obtaining carbon nanofibers in the laboratory may not be feasible from an industrial standpoint due to the limited manufacturing capacity.
In the case of the floating catalyst technique, the reaction takes place in a specific volume without the metallic catalyst particle being deposited on any substrate surface, since it is introduced in the reactor in a continuous manner suspended in the reacting gas flow. The advantage of this technique is that there is no need to later take apart the nanofibers produced from the substrate. In this way, as the supply of reagents to the process furnace and the collection of the product obtained are continuous, the process for producing carbon nanofibers using the floating catalyst technique is directly applicable in industry, unlike carbon nanofiber production processes based on techniques in which the catalyst particles are deposited on a substrate that is subsequently introduced into the process furnace to activate the formation of the carbon nanofibers and eventually removed to collect these carbon nanofibers, separating them from the substrate on which they have been grown.
The carbon nanofibers of the present invention are formed from the metallic catalyst particles suspended in the process furnace gas flow, forming nanometric graphitic fibrillar structures.
The nanofiber graphitic filaments continue growing until the catalyst particles are poisoned or over-saturated with carbon. After the graphitic filament has been grown, a filament thickening process would take place that involves the deposition of pyrolytic carbon on the carbon nanofiber surface, which presents lower structural order than the catalytic graphitic carbon.
Hereinafter said pyrolytic carbon will be identified as amorphous carbon. Amorphous carbon has negative effects on the surface activity of the nanofiber, as will be commented further below, and therefore also has negative effects on its possible applications.
There are studies, such as those by Oberlin [Oberlin A. et al., Journal of Crystal Growth 32, 335 (1976)] that analyze the growth of carbon filaments on metallic catalyst particles by transmission electron microscopy techniques.
Based on these studies, Oberlin proposed a growth model for carbon nanofibers or nanofilaments based on diffusion of carbon about the surface of the catalyst particles until the surface of these particles is saturated or poisoned by excess carbon.
Oberlin also explained that the deposition of amorphous carbon by thermal pyrolytic decomposition is the process responsible for the thickening of graphitic filaments previously grown from the metallic catalyst, and that said pyrolytic process takes place whenever the temperature of the process furnace is high enough and the residence time of the carbon nanofibers in the process furnace is long enough. Therefore, after the catalytic carbon filament growth process has concluded due to the poisoning or the carbon saturation of the catalyst particle, the filament continues to be thickened if it remains exposed to the pyrolysis conditions for an extended period.
The carbon nanofiber thickening process due to the deposition of amorphous carbon of pyrolytic origin is very difficult to avoid, due to the fact that the deposition of pyrolytic amorphous carbon on the carbon nanofiber surface takes place very quickly at production conditions on a floating catalyst system. Thus, only in the case of very low residence time of the gaseous furnace atmosphere, which transports along the furnace the catalyst particles and the carbon nanofibers produced in the process, is possible to avoid the deposition of pyrolytic amorphous carbon on the carbon nanofibers surface, thereby avoiding the loss of quality and properties of the carbon nanofibers resulting in lower graphitization degree, lower specific surface area value and lower specific volume of mesopores.
Considering the structure of the carbon nanofiber graphitic filament of catalytic origin, several configuration models have been identified to describe its graphitic structure based on the different possible growth forms depending on the metallic catalyst particles and the reaction conditions. In some cases the carbon nanofibers are solid and are configured by superimposed graphitic stacked flat plates (known as a platelet structure) and other times their structures are more complex presenting the well known fishbone structure. In the case of not stacked graphitic planes, it is possible to differentiate either a hollow or solid structure composed by the planes in the form of superimposed ribbons along the axis of the carbon nanofibers (known as a ribbon structure). Finally there is other possible structure in the form of stacked truncated cones (stacked cup structure).
The structure of carbon nanofibers is often modelled by one of these configurations, although in most cases it is difficult to precisely determine the actual structure due to the limitations of the inspection and analysis instruments. This is why we refer to “modelling”, as it is understood that there is a reasonable fit with one of the aforementioned structures.
FIGS. 3 and 4 are transmission electron microscopy images of a carbon nanofiber according to an embodiment of the invention showing the end of a carbon nanofiber wherein its spiral ribbon is partially unwounded showing a periodically twisted structure. These figures will be used in the detailed description of the invention.
In our case it is not only a model, as the experiments conducted confirm the structure of the nanofiber, as the pictures of FIGS. 3 and 4 clearly show.
There is no doubt that the structure of the carbon nanofibers determines their physical properties at a macroscopic level when used in industrial applications.
For example, structures consisting of disjointed planes would result in a lower electrical conductivity of the filament than in structures which graphitic planes are continuous along the fiber axis providing a high conductivity.
A similar argument applies to the specific surface area of the carbon nanofibers. The free edges of the basal graphitic planes formed during the filament growth are important in all of these carbon nanofiber properties. These free edges increase the specific surface area of the carbon nanofibers and consequently they favour gas adsorption and interaction with other substances to form chemical bonding.
If the external structure of the carbon nanofiber, and more specifically the basal plane free edges, are covered by pyrolytic amorphous carbon then this forms a passivating barrier that hinders the chemical activity of the carbon nanofibers, reducing their capacity to interact with other substances or molecules and reducing the final specific surface area. In this case the fiber will have a poorer quality and fewer applications.
Similarly, a greater presence of pyrolytic amorphous carbon in the carbon nanofibers implies a reduced graphitization degree, which is closely related to their physical properties such as the thermal and electric conductivities; in short, it affects the final quality of the carbon nanofibers and their possible applications.
U.S. Pat. No. 5,024,818 is known, which describes a method for producing carbon nanofibers from carbonaceous compounds. In this patent it is specified that Fe is used as a catalyst. The furnace described in this patent is a floating catalysis process furnace.
Based on the data supplied in the patent, it can be inferred that the furnace operates from 1100° C. to 1150° C. (around 1140° C.), and that it uses a mixture of Fe compounds and S compounds with a molar ratio of 1/1.
Similarly, it can also be inferred that the residence time of the gaseous reagents under the process conditions inside the furnace described in this patent is about 30 s, with a travel velocity of these gases from 0.011 m/s to 0.033 m/s.
The gas stream velocity inside the furnace is important from a production standpoint. This gas stream velocity inside the furnace is directly related to the ratio [L0]/[t0], where [L0] is the characteristic length of the furnace and [t0] is the characteristic residence time of the gaseous mixture in the furnace.
The production capacity is determined by the [L0] of the process furnace, by the gas stream velocity and by the residence time [t0] needed. The larger the [L0] of the furnace, the higher the gas stream velocity and the shorter the residence time [t0] are the greater the production capacity of the furnace process is.
The minimum residence time [t0] needed is determined by the time required for nucleation and growth of the carbon nanofibers. The characteristic furnace length [L0] is mainly constrained by constructive limitations derived from limited features of the materials currently available for manufacturing such kind of furnaces.
The need for a high [L0] would lead to a greater size of the furnace that can make its construction unfeasible. Current techniques of making this hind of furnaces do not allow exceeding certain size of the furnace.
These are the main physical limitations, so that it is of interest to act on the residence time [t0] required for the carbon nanofiber nucleation and growth allowing this way to increase the average circulation velocity inside the furnace and therefore increasing the production capacity of the furnace accordingly.
However the circulation velocity cannot be increased arbitrarily. The aforementioned U.S. Pat. No. 5,024,818 sets a limit for the velocity of the gas stream carrying the Fe catalyst particles to avoid turbulent regimes, as otherwise the fluctuations resulting from the vorticity will prevent a stable growth. This patent establishes for one of the preferred embodiments a circulation velocity of 0.033 m/s.
The object of this patent is to determine the conditions of the reaction process in which the average stream velocity exceeds even by several orders of magnitude those velocities used in U.S. Pat. No. 5,024,818 patent, obtaining accordingly a much higher production capacity and therefore significantly increasing its industrial applicability.
Also the object of this patent is the carbon nanofibers obtained with improved properties, particularly their specific surface area, their graphitization degree and their specific volume of mesopores, thereby improving their gas adsorption capacity, their physical properties and finally their overall quality, allowing a wide industrial applicability.